The polymerization of acrylic acid in solution usually takes place in a temperature range from 20.degree.-100.degree. C. The chemical reactivity of acrylic aid, primarily at higher temperatures, leads to the fact that practically all the material is attacked sooner or later by the acrylic acid and the polymer gel adheres firmly to this material. This adhesion of the polymer gel is even stronger if the polymerization takes place quickly and there is thus little time for cooling and the removal of the heat of the reaction and a reduction in volume of the polymer gel.
A number of processes for the production of polymers and copolymers of water-soluble monomers such as acrylic acid or methacrylic acid are known, although, in every instance, these are unsatisfactory from one point of view or another.
In the process and apparatus described in DE-OS 20 59 241, mold boxes of various shapes and heights, with capacities varying from 2-200 liters, are transported on a linked belt conveyor and filled with the monomer solution and the catalytic solutions from a dispensing apparatus, through a mixing apparatus, with the help of metering pumps. This means that a monomer solution can also be polymerized even at increased layer thicknesses, although the advantages of producing an endless strand of polymer gel cannot be realized by this method. The mold boxes can be of thermoplastic material or of other material--for example, metal--that is coated or provided with a layer of thermoplastic material. In this regard, the weight of the polymer gel must be such that the contact area A between the gel mass and the inside wall of the mold box, the adhesive force F and the gel weight W satisfy the relationship: W&gt;A.F. (see DE-AS 24 21 006).
Even when this condition is satisfied, small amounts of polymer gel remain in the mold box, due to the high viscosity and tackiness of the polymer gel, and these have to be removed. In this process, cleaning the individual mold boxes is both difficult and time consuming since it is impossible to provide for continuous cleaning by means of rotating brushes. In order to simplify the removal of the polymer gel from the mold boxes, various shapes of polymerization vessels have been experimented with; a preferred shape is described in DE-OS 28 24 313, this being in the form of a truncated cone, a hemisphere, or a truncated pyramid.
In addition to linked belt conveyors with containers, endless belt systems with endlessly moving flat conveyor belts are known for carrying out the polymerization process. A redox-system polymerization of water soluble polymers on an endless conveyor belt is described, for example, in U.S. Pat. No. 4,138,539. In this process, the monomer solution is mixed with azoisobutyronitrile and ammonium iron sulphate and sprayed onto the belt at a metered rate of 2 kg per minute. At a belt speed of 1 foot/88 seconds, the total polymerization time amounts to 65 minutes.
Polymerization can also be initiated by high-energy radiation with mercury vapour or xenon lamps as well as by fluorescent lights, as is described in DE-OS 20 50 988 or DE-OS 20 09 748. In this process, the monomer solution with the dissolved photoinitiator usually flows through an applicator system onto an endless steel belt or onto a conveyor belt with a carrier of water repellent material of, for example, fluoropolyolefin polymer or copolymer or metal coated plastic foil, which makes it possible to scrape off the dried polymer.
In the production of an emulsion of a water soluble polymer by means of ultraviolet light on a moving conveyor belt, coating thicknesses or 10-50 mm are used, whereby a viscous polymer emulsion results as the end product, as is described, for example, in DE-OS 32 08 369.
For the production of polymers of, for example, (meth)acrylic acid, on an endless conveyor belt, the known conveyor belt designs that are used, having guide pulleys and optional tensioning systems, are essentially flat and have no pre-formed trough which would permit a large coating thickness of the applied polymer solution. Nor does the conveyor belt assume a trough-like shape, because of the relatively low weight of the liquid reaction components, which are applied in only a thin layer. For this reason, the throughput and space-time yield for the polymers that are produced are relatively low in the case of such known continuous processes. In actual fact, in most cases, coating thicknesses of only 0.5 to a maximum of 2 cm. have been reported--see DE-OS 2545 290, DE-OS 27 16 606, DE-AS 10 32 922, U.S. Pat. No. 3,929,751, DE-AS 22 48 715, DE-OS 20 50 988, and DE-OS 35 06 534.
Only DE-OS 32 46 905 reports a possible coating thickness of 1-10 cm for a polymerization time of 1-24 hours and a subsequent storage time of the polymer gel of 10-50 hours for the production of methacrylamide polymers on a thermally controlled base (for cooling) having a release sheet on an endless belt system. However, no construction features of the belt system which would permit such a thick layer of liquid reaction components are described at all. In the case of such a thick layer of reactant solution, it is necessary to have a sealing capability in all directions so as to prevent the solution from running off or flowing back. As far as can be ascertained from the examples, a reaction time of 2 hours for a 2 cm layer and obviously correspondingly longer times (of up to 24 hours) for greater coating thicknesses indicate that the space-time yield for these polymerization processes is very low.
In order to counteract the tackiness of the resulting polymer gels on the wall of the polymerization vessels, as in the case of the linked belt conveyors described above, attempts have been made to work with the addition of a thermally decomposed free-radical initiator (see DE-OS 22 48 715) or by the addition of higher aliphatic acids and their salts, lauric, myristic, palmitic, stearic, arachidic or behenic acid according to DE-OS 28 24 313 or DE-AS 27 47 168.